EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 



79 



In an embryo of 23 somites, 2.5 mm. long, described by Thompson, 

 the allantois has elongated and shows three irregular dilatations (Fig. 80). 

 A large cavity never appears distally in the human allantois as in Ungu- 

 lates, but when it becomes included in the umbilical cord its distal portion 

 is tubular. The allantois eventually atrophies and is without further 

 significance (cf. p. 209). 



The human allantois is thus small and rudimentary as compared with 

 that of birds and Ungulates. As we have seen, the cavity is very large 



in the pig, and Haller found an 

 allantoic sac two feet long con- 

 Phar nx 7 ***^ \ nected w ^h a goat embryo of 



Pharyngeal 

 membrane 

 Thyreoid 



Pericardium 



Hepatic diverticulum 

 Septum transversum 



Cloacal membrane 

 Cloaca 



Neural folds 



Neurenteric canal 



FIG. 80. Median sagittal section of a 2.5 mm. FIG. 81. Human embryo of 2.11 mm. 

 human embryo, showing digestive tract (after (Eternod). X 35. 



Thompson). X_4O. 



two inches. In human embryos it appears very early and is not free, 

 but embedded in the body stalk. Its functions, so important in birds and 

 Ungulates, are in man performed by the chorion. 



The Yolk Sac and Stalk. In the youngest human embryos described, 

 the entoderm forms, a somewhat elongated vesicle (Fig. 76). With the 

 development of the fore-gut and hind-gut in embryos of 1.54 and 2 mm. 

 (Figs. 77 and 79), the entodermal vesicle is divided into the dorsal intestine 

 and ventral yolk sac, the two being connected by a somewhat narrower 

 region. This condition persists in an embryo 2.5 mm. long (Fig. 80). 

 In the figure, most of the yolk sac has been cut away. Embryos with 



